Yoshinori Uzawa

Superconducting properties and crystal structures of single‐crystal niobium nitride thin films deposited at ambient substrate temperature

Single‐crystal niobium nitride (NbN) thin films were fabricated at ambient substrate temperature so that photoresist lift‐off techniques could be used in fabricating Josephson tunnel junctions. In this article, we describe the superconducting properties and crystal structure of the NbN films. Even though the substrates were not heated, the NbN films had excellent superconducting properties: a high Tc of 16 K, low normal‐state resistivity (ρ20=62 μΩ cm), and residual resistivity ratios RRR=ρ300/ρ20 above one. The film structures, which were investigated by x‐ray diffraction, electron diffraction patterns and transmission electron micrographs, show a single‐phase orientation without columnar and granular structures. We have found that the superconducting properties depend on the lattice parameter, and the best films had a lattice parameter of 0.446 nm. NbN/AlN/Nb tunnel junctions were fabricated to measure the superconducting energy gap of the NbN films. We estimated the energy gap ΔNbN to be 2.6 meV and th...

Light-induced collective pseudospin precession resonating with Higgs mode in a superconductor

Optically manipulating superconductors In superconductors, electrons of opposite momenta pair to form a highly correlated state that manages to flow without encountering any resistance. Matsunaga et al. manipulated the wavefunction of these pairs in the superconductor NbN with an electromagnetic pulse that they transmitted through a thin layer of the material (see the Perspective by Pashkin and Leitenstorfer). The superconducting gap, which is the energy needed to break the pairs apart, oscillated at twice the frequency of the pulses electric field. When they matched this frequency to half the gap, the authors excited a collective mode in the superconductor called the Higgs mode, a relative of the Higgs boson in particle physics. Science, this issue p. 1145; see also p. 1121 Terahertz pump-probe spectroscopy reveals the oscillations of a superconducting order parameter at twice the pump frequency. [Also see Perspective by Pashkin and Leitenstorfer] Superconductors host collective modes that can be manipulated with light. We show that a strong terahertz light field can induce oscillations of the superconducting order parameter in NbN with twice the frequency of the terahertz field. The result can be captured as a collective precession of Anderson’s pseudospins in ac driving fields. A resonance between the field and the Higgs amplitude mode of the superconductor then results in large terahertz third-harmonic generation. The method we present here paves a way toward nonlinear quantum optics in superconductors with driving the pseudospins collectively and can be potentially extended to exotic superconductors for shedding light on the character of order parameters and their coupling to other degrees of freedom.

Higgs amplitude mode in the BCS superconductors Nb1-xTi(x)N induced by terahertz pulse excitation.

Ultrafast responses of BCS superconductor Nb(1-x)Ti(x)N films in a nonadiabatic excitation regime were investigated by using terahertz (THz) pump-THz probe spectroscopy. After an instantaneous excitation with the monocycle THz pump pulse, a transient oscillation emerges in the electromagnetic response in the BCS gap energy region. The oscillation frequency coincides with the asymptotic value of the BCS gap energy, indicating the appearance of the theoretically anticipated collective amplitude mode of the order parameter, namely the Higgs amplitude mode. Our result opens a new pathway to the ultrafast manipulation of the superconducting order parameter by optical means.

HIGH CRITICAL CURRENT DENSITY NBN/ALN/NBN TUNNEL JUNCTIONS FABRICATED ON AMBIENT TEMPERATURE MGO SUBSTRATES

NbN/AlN/NbN tunnel junctions are fabricated at ambient temperature on MgO substrates, and a critical current density of 8 kA/cm2 is obtained in junctions with 1.5–nm–thick AlN barriers. Even though the NbN/AlN/NbN trilayers are deposited without intentional heating, the junctions show a large gap voltage (Vg=5 mV), sharp quasiparticle current rise (ΔVg=0.16 mV), and small subgap leakage current (Vm=25 mV and Rsg/RN=9). This report shows that high‐quality NbN/AlN/NbN tunnel junctions can be made at ambient substrate temperature.

Interface and tunneling barrier heights of NbN/AlN/NbN tunnel junctions

The tunneling barrier height of NbN/AlN/NbN tunnel junctions was measured by investigating the barrier thickness dependence of the current density, and the junction interface was studied by cross-sectional transmission electron microscopy (TEM). We found that the current density of the junctions has two distinct types of dependency on the AlN barrier thickness, corresponding to two average barrier heights in different regions for the current density. The TEM observations showed that the junctions had a very smooth and clear electrode–barrier interface, and the crystal structures of the counterelectrode NbN films were strongly dependent on the thickness of AlN barriers. The average barrier height was estimated to be 2.35 eV in the low-Jc region, Jc 5 kA/cm2.

Terahertz NbN/AlN/NbN mixers with Al/SiO/NbN microstrip tuning circuits

We have developed a low-noise quasioptical NbN/AlN/NbN superconductor-insulator-superconductor (SIS) mixer that operates at terahertz frequencies. The mixer uses a MgO hyperhemispherical lens with an antireflection cap, a single-crystal NbN log-periodic antenna, and two-junction tuning circuits which employ Al/SiO/NbN microstriplines. The NbN/AlN/NbN junction size was about 0.9 μm in diameter, and the current density was about 45 kA/cm2. The frequency dependence of the receiver noise temperature was investigated by using an optically pumped far-infrared laser and a backward-wave oscillator as a local oscillator at several frequencies from 670 to 1082 GHz. The experimental results showed that the center frequency was around 800 GHz, and the receiver noise temperature measured by the standard Y-factor method was 457 K (DSB) at 783 GHz including a 9-μm-thick Mylar beam splitter loss and other optical losses at the physical bath temperature of 4.2 K. This is the first SIS mixer based on NbN with low-noise per...

NbN/AlN/NbN tunnel junctions with high current density up to 54 kA/cm2

We report on progress in the development of high current density NbN/AlN/NbN tunnel junctions for applications as submillimeter wave superconductor-insulator-superconductor mixers. A very high current density up to 54 kA/cm2, roughly an order of magnitude larger than any reported results for all-NbN tunnel junctions, was achieved in the junctions with about 1 nm thick AlN barriers. The magnetic field and temperature dependence of critical supercurrents were measured to investigate the Josephson tunneling behavior of critical supercurrents in the high-Jc junctions. The junctions showed high-quality junction characteristics with a large gap voltage of 5 mV and sharp quasiparticle current rise (ΔVg=0.1 mV). The Rsg/RN ratio was about 5 with a Vm value of 14 mV measured at 4.2 K.

Development of epitaxial NbN/MgO/NbN–superconductor-insulator-superconductor mixers for operations over the Nb gap frequency

We have developed fabrication processes for epitaxial NbN/MgO/NbN trilayers and epitaxial NbN/MgO/NbN tunnel junctions. We estimated the junction specific capacitances (CS) for designing fully epitaxial NbN–superconductor-insulator-superconductor (SIS) mixers by using Fiske steps. Tunnel junctions measuring 40×3 and 20×3 μm2 were fabricated, and Fiske steps caused by external magnetic fields were clearly observed in the I–V characteristics. The CS was estimated as a function of the product of the normal-state resistance and the junction area (RNA), and they were 73–120 fF/μm2 at 8800–90 Ω μm2. We fabricated and tested a fully epitaxial NbN/MgO/NbN–SIS mixer. The mixer showed low-noise properties over the Nb gap frequency, and the lowest DSB receiver noise was 260 K at 795 GHz.

Performance of all-NbN quasi-optical SIS mixers for the terahertz band

We have designed, fabricated, and tested terahertz superconductor-insulator-superconductor (SIS) mixers having a self-compensated NbN/AlN/NbN tunnel junction and an epitaxially grown NbN/MgO/NbN microstripline. The junction as a distributed element was 0.8-/spl mu/m wide and 2.4-/spl mu/m long, and its current density was 50 kA/cm/sup 2/. The microstripline consisted of a 200-nm-thick NbN ground plane, a 180-nm-thick MgO insulator, and a 350-nm-thick NbN wiring layer. An investigation of the mixer in our receiver configuration showed flat noise characteristics from 870 to 960 GHz. The lowest receiver noise temperature of about 550 K was obtained at 909 GHz, including a 9-/spl mu/m-thick Mylar beam splitter loss and other optical losses. These characteristics suggest that SIS mixers with self-compensated NbN/AlN/NbN junctions and NbN/MgO/NbN microstriplines are appropriate for used in wideband and low-noise operations at terahertz frequencies.

IF bandwidth and noise temperature measurements of NbN HEB mixers on MgO substrates

We report the fabrication and testing of hot electron bolometric mixers with an ultrathin NbN film as heterodyne receivers operating at terahertz frequencies. We found that the quality of the NbN strip is usually degraded by damage resulting from the fabrication process, and we developed a process for reducing such damage. We also fabricated HEB mixers with a new structure in order to study the IF bandwidth determined by the original quality of the NbN thin films. Investigations at 100 GHz revealed that the widest IF bandwidth of 2.0 GHz was obtained by a mixer based on a 2.8 nm-thick NbN film. An HEB mixer based on the 2.8 nm-thick NbN film was also fabricated and evaluated. The receiver noise temperature at 900 GHz was 780 K, the absorbed LO power was about 400 nW, and the conversion gain was -13 dB.